Recently, as the use of electronic devices, such as a mobile phone, a portable notebook computer, an electric vehicle, and the like, using a battery, is rapidly spreading, demand for a lithium secondary battery which is small and light yet has a relatively high capacity is increasing, and this trend continues to accelerate.
Natural graphite, commonly used as a core carbon material of a negative active material (or an anode active material) for a lithium secondary battery, has an excellent initial discharge capacity but has a problem in that as a charging and discharging cycle is repeated, the charging and discharging efficiency and charging and discharging capacity thereof are drastically degraded. Such a problem results from electrolyte decomposition generated from an edge portion of the highly crystalline natural graphite.
Thus, in an effort to overcome this problem, a technique of coating natural graphite with pitch is commonly used. In addition, in manufacturing an electrode plate of a battery after a negative active material is formed by coating natural graphic with pitch, a method of adding a conductive material such as Ketjen Black, Super P, or the like to slurry in use for manufacturing the electrode plate is also used in order to improve electrical conductivity.
Meanwhile, recently, a technique of improving conductivity by adding a conductive additive such as carbon fibers to a negative electrode for a lithium secondary battery using natural graphite has been introduced. In relation to this, PCT/JP 2004-019835 discloses a technique in which, after a carbonaceous negative active material is completely manufactured, a binder such as styrene-butadiene rubber (SBR) and carbon fibers having a diameter of 1 nm to 1000 nm are mixed and subjected to roll mixing milling, the end product of which is then coated on a metal current collection plate to thus constitute a negative electrode for a lithium secondary battery.
However, this related art has a problem in that carbon fibers are not evenly distributed within the negative active material for a lithium secondary battery and are re-agglomerated so as to degrade the electrochemical characteristics thereof. In addition, it appears that the use of SBR as an electrode binder makes it difficult for the hydrophobic carbon fibers to be sufficiently distributed within the negative active material.